Given the current trend in medicine—dictated by the requirement of national health services and private insurance companies to limit medication costs—for carrying out the greatest possible number of surgical operations without general anaesthetic and therefore without patient hospitalisation, there is current interest in further developing local and regional, rather than general, anaesthetic is techniques. Given recent developments and the comparative progress achieved, regional and local anaesthesias prove to be even safer than general anaesthesia, which is hence avoided particularly in vulnerable patients such as the elderly. It is therefore a problem not only of cost to be borne by the community, but also above all of improving the quality of therapy offered to the patient, which improves compliance thereof.
The aforementioned so-called regional anaesthesias include, as a rule, techniques suitable for administering local anaesthetics to the spine and the nerve plexus of the upper extremities, as well as to the individual peripheral nerves. Spinal techniques are divided in their turn into epidural injection and intrathecal injection (whereby the narcotic is injected into the so-called subarachnoid space) which are both suitable for inducing, by means of a targeted anaesthetic injection into a contained spinal space, a regional anaesthesia of the lower extremities by temporarily interrupting the nervous connection between said extremities and the brain. While the intrathecal technique is more invasive than the epidural technique (in that the injection is carried out in a region deeper within the spine), it has the advantage of requiring comparatively small quantities of the anaesthetic used.
An ideal intrathecal anaesthetic for outpatient surgery use should give an immediate or at least a rapid effect (and thus have a brief induction period—so called onset time), should have an easily adjustable action for a predictable duration, and should exhibit low neurotoxicity as well as be without side effects. A narcotic already in use for intrathecal anaesthesia is procaine, which can however result in an inadequacy rate of 17% (1). Instead, lidocaine has been associated with symptoms of so-called transient neurologic syndrome (TNS) (2), whereas bupivacaine though efficient, can nevertheless induce blocks whose duration in some cases is hard to predict despite low dose administration.
It therefore appears that none of the anaesthetics currently used for intrathecal application, in currently authorized formulations, fully satisfies all the criteria that characterize an ideal preparation. The need remains therefore to provide additional and improved compositions for intrathecal administration.